1. Field of the Invention
The present invention relates to a color image pickup apparatus such as a still video camera and a video camera.
2. Related Background Art
Hitherto, in order to frame-photograph (obtain odd and even field information items) a still image by a still video camera or the like, there has been a necessity of using a color filter image pickup device having a color filter which is operated at a period of two pixels as shown in FIGS. 1A and 1B. The reason for this is in that it is necessary to make the outputs from the odd fields and even fields the same because the conventional image pickup device alternately reads the odd rows and even rows as shown in FIGS. 1A and 1B.
However, if the image pickup device having the color filter arranged as shown in FIGS. 1A and 1B is used, a luminance signal and a chrominance signal can be deduced by using data of two rows positioned alternately, for example, the first and third rows in the odd field. The second and fourth rows in the even field must be used. As a result, the vertical directional distance between data items is enlarged, causing the correlation in the vertical direction to be reduced. Therefore, the quantity of generation of pseudo color undesirably increases.
Another problem arises in that a multiplicity of 1H (horizontal scanning period) delay lines are necessary to deduce the luminance signal and the chrominance signal for use in the signal process.
FIG. 2 illustrates the positions at which carrier components are generated in the vicinity of the base band of an image pickup device provided with the above-described color filter. In either of the cases shown in FIGS. IA and IB, the carrier of the luminance signal is generated at positions (.+-.1/PH, 0) assuming that the horizontal scanning directional pitch of the color filters of the image pickup device is PH. Furthermore, the carrier of the chrominance signal is generated at positions (.+-.1/2PH, 0) because the pitch of the filters of the same color disposed in the horizontal direction is 2PH.
The luminance signal obtainable from the above-described image pickup device is arranged to have its band to a frequency of 1/2PH in accordance with the theorem of sampling. Therefore, in order to prevent the reflected distortion of the luminance signal, it is ideal that the optical low-pass filter to be disposed in front of the image pickup device is able to make all of the frequency components higher than 1/2PH to be zero. However, the carrier of the chrominance signal is generated at the horizontal frequency fH=.+-.1/2PH as shown in FIG. 3. As a result, if the optical low-pass filter having the above-described frequency characteristics is used, the pseudo color is generated due to the reflection of the chrominance signal. Therefore, as shown in FIG. 3, the frequency characteristics of the optical low-pass filter must have the band which is lower than fH=1/2P.sub.H by a degree corresponding to the chrominance signal. As a result, a problem arises in that the obtainable resolution is lower than the principle resolution limit.
FIG. 4A illustrates the structure of a conventional optical low-pass filter for use in a solid image pickup device in which the color filters are arranged as shown in FIG. 1. Referring to FIG. 4A, an optical low-pass filter 70 comprises a birefringence plate 71 for dividing a light beam into two beams, the light beam being a beam made incident upon by making an angle of 90.degree. from the horizontal direction by a distance of PH. The low-pass filter 70 further comprises a phase plate 72 for converting linearly polarized light into circularly polarized light. The low-pass filter 70 further comprises another birefringence plate 73 for dividing a light beam into two beams, the light beam being a beam made incident upon by making an angle of 90.degree. from the horizontal direction by a distance of PH/2. The transfer characteristic (MTF) H1 of the above-described optical low-pass filter is expressed by the following equation: EQU H1 (fx, fy)=.vertline.cos (.pi./2 PH fx).multidot.cos (.pi.PH fx).vertline.(1)
The above-described characteristic can be graphed as shown in FIG. 4B. The characteristic on the frequency space is as shown in FIG. 4C. Dotted lines 74a, 74b, 75a and 75b show the frequencies at which the optical low-pass filter 70 traps. As can be seen from FIG. 4C, carrier frequency fH=.+-.1/PH of the luminance signal and carrier frequency fH=.+-.1/2PH of the chrominance signal are trapped. As can be seen from Equation (1) or FIG. 4B, the optical low-pass filter having the above-described characteristics is as follows: the MTF is 0 at the carrier frequency fH=.+-.1/2PH of the chrominance signal; and the transference characteristic is-15 dB at about 85 of the frequency. Therefore, the number of the resolution lines to the resolution limit fH=1/2PH cannot be obtained.